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Final Report Summary - NIDYFICS (NIckel DYnamics in impacted ultramaFIC Soils)

Pyrometallurgical process of metallic ores generates large amounts of wastes, considered as hazardous materials, as they contain potentially toxic elements, like nickel. Wastes are reused for construction or reprocessed, but a significant fraction is commonly dumped on soil surface or into lakes, where their dispersal by wind or water may have environmental impact on large areas. Another way of reuse could be in agronomy as soil additive. The purpose of the NIDYFICS project (Nickel Dynamics in impacted ultramaFIC Soils) is to characterize the dynamics of nickel and other associated metals in the soil-waste-plant-water continuum in the ultramafic massifs of Barro Alto and Niquelândia (Goïás, Brazil), which are considered as a hotspot of biodiversity. Thus, it is crucial to unravel their solid speciation, which controls potential release and mobility of these contaminants and to assess the environmental impact of wastes dispersal in the lateritic soils. Moreover, to understand and quantify the metal dynamics, the measurement of metal isotopic ratios may be a key tool to trace both their sources and fate in the environment. Finally, the benefits and also the risk of metal contamination associated to the waste reutilization in the sugar cane production need to be assessed at both laboratory and field scales.
This multidisciplinary project involves soil scientists, spectroscopists, geochemists and agronomists, who have already started to work in synergy on this topic for several years. This joint research and exchange project has led to the acquisition of basic results about Ni isotopic geochemistry. Ratié et al. (2015) have shown that lateritic weathering leads to heavier Ni isotope depletion in the solid phase (Δ60NiSoil-Bedrock = -0.47‰), and that the formation of Ni-bearing clay minerals and Fe-oxides appeared to lead to depletion in heavier Ni isotopes, which indicates preferential export of heavy isotopes in the dissolved phase. Nickel isotope ratios were also measured in ores, fly ash, slags and ferronickel (FeNi) samples from two metallurgical plants, in order to investigate the mass-dependent fractionation of Ni isotopes during the Ni-laterite ore smelting and refining (Ratié et al., 2016). Feeding material exhibits a large range of δ60Ni values (0.02 ± 0.10‰ to 0.20 ± 0.05‰), explained by the diversity of Ni-bearing phases. Both δ60Ni values of fly ash (0.07 ± 0.07‰, n=10) and final FeNi produced (0.05 ± 0.02‰, n=2) were not significantly different from the feeding materials ones, in line with the very high production yield of the factories. Smelting slags present heavier δ60Ni values, ranging from 0.11 ± 0.05‰ to 0.27 ± 0.05‰ (n=8). Soils were also collected near and far from the Niquelândia metallurgical plant, to evaluate the potential of Ni isotopes for tracing the natural vs anthropogenic Ni in soils. The slight but significant enrichment in heavy isotopes compared to soils highlights the potential impact of smelting activity in the surrounding area, as well as the potential of Ni isotopes for discerning anthropogenic samples (heavier δ60Ni values) from natural ones (lighter δ60Ni values). However, given the global range of published δ60Ni values (from -1.03 to 2.5‰) and more particularly those associated to natural weathering of ultramafic rocks (from -0.61 to 0.32‰), the use of Ni isotopes for tracing environmental contamination from smelters will remain challenging.
The mineralogical and geochemical characterization of the major smelting residues from Barro Alto and Niquelândia smelters (slags and fly ash) was performed to characterize the concentration of Ni and other contaminants and the role of main hosting-phases. Slags produced by the quenching process at 1500°C contain relatively low levels of Ni (779 mg.kg-1) compared to fly ash produced by the laterite ore calcination at 800°C (25100 mg.kg-1). In contrast, in agreement with refractory behavior of Cr, slightly higher Cr concentrations were found in slags than in the fly ash (7580 mg.kg-1and 1870 mg.kg-1, respectively). To evaluate the leaching properties and potential risk related to the leaching of Ni and other contaminants, we used standard EU leaching procedures for waste materials coupled to speciation-solubility modeling using PHREEQC-3 and investigation of newly formed phases (Ettler et al., 2016). Granulated slag dusts and fly ash samples were also investigated with a special focus on the in vitro bioaccessibility in a simulated gastric fluid, to assess the potential exposure risk for humans (Ettler et al., in press, 2017). Bioaccessible fractions (BAFs) of the major contaminants (Ni, Co, and Cr) were quite low for the slag dusts and accounted for less than 2 % of total concentrations. In contrast, BAF values were significantly higher for fly ash materials, which reached 13 % for Ni and 19 % for Co. Results indicate that there is only a limited risk to human health related to the ingestion of dust materials generated by laterite Ni ore smelting operations if appropriate safety measures are adopted at the waste disposal sites and within the smelter facility.
Pot and column experiments have been performed to understand potential effects of slag addition to local soils and improvement of their fertility (in terms of Mg and other nutrients).
The aim of the NIDYFICS project was also to connect researchers in environmental geosciences for high level discussions and exchanges and for transfer of knowledge between European and Brazilian institutions. In that context, a workshop entitled “Environmental impacts of mining and smelting” was organized at Orsay, the January 8-9th 2015 (www.minmetpoll.u-psud.fr). It was jointly organized by the NIDYFICS partners (University Paris Sud, IPGP, Charles University in Prague and Brasilia University) together with the SOLEIL synchrotron facility. Sixty-five persons attended this two-day workshop, coming from Europe, USA, Africa and Brazil, among them one third of PhD or MSc students. The organizers were proud to invite experts in the field of geochemistry in mining and smelting environments: the keynote lectures have been given by Nadine Piatak (USGS, Reston, USA), Guillaume Morin (Institut de Minéralogie, Physique des Matériaux et Cosmochimie, Université Pierre et Marie Curie, UPMC, Paris, France), Jeroen Sonke (GET – Géosciences Environnement Toulouse, France) and Luiz de Andrade Lima (University of Bahia, Brazil). Selected papers have been published in a special issue of the Applied Geochemistry special issue on Environmental impacts of mining and smelting (64, 2016).
A key point of NIDYFICS was the training of young researchers in a promising research field by working with and sharing knowledge with experts. Therefore, 4 PhD students have been directly or indirectly involved in the NIDYFICS project, as well as several master students.

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UNIVERSITE PARIS-SUD
France
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